Lecture 5 medium

Smooth ER

• Located near the cell periphery, the smooth endoplasmic reticulum (SER) lacks ribosomes, distinguishing it from rough ER.
• Functions include lipid, steroid, and carbohydrate synthesis, along with metabolizing drugs and toxins.
• The type of cell influences the specific functions of the SER.
• In muscle cells, it's known as the sarcoplasmic reticulum, regulating calcium ion concentrations during muscle contraction and relaxation.
• In adrenal glands, SER secretes cholesterol, a precursor to steroid hormones.
• In liver cells, SER synthesizes and stores detoxifying enzymes to process harmful substances like alcohol and drugs.

Functions of ER

• Rough ER (RER) is primarily responsible for protein production, while SER synthesizes phospholipids and steroids.
• The ER detoxifies harmful exogenous molecules and regulates calcium ion sequestration and release.

Lysosomes

• Heterophagia involves breaking down foreign substances absorbed via endocytosis.
• Autophagy is the process of degrading internal materials, including nutrients and dysfunctional organelles.
• Autolysis occurs when lysosomes rupture, digesting the cell due to pathology or aging.
• Primary lysosomes form from the Golgi apparatus and contain inactive hydrolases for degrading large molecules.
• Secondary lysosomes form when primary lysosomes merge with endocytic vesicles, digesting various materials and recycling products to the cytoplasm.

Endoplasmic Reticulum Structure and Function

• The endoplasmic reticulum (ER) produces, processes, and secretes proteins, playing a vital role in every cell of humans and other organisms.
• Its structure consists of interconnected flattened sheets (cisternae) and tubules, facilitating efficient function.
• Rough ER is studded with ribosomes, while the smooth ER lacks them.

Rough ER

• Lumen of rough ER is narrower than that of smooth ER.
• Central to protein synthesis as ribosomes translate mRNA into proteins, which are then modified and transported.

Golgi Network

• Composed of two networks: cis and trans Golgi networks, each made of multiple cisternae.
• The cis Golgi network is adjacent to the ER, while the trans Golgi network is the farthest.
• The Golgi modifies proteins and lipids received from the ER, packaging them into vesicles for transport.

Protein Trafficking

• Newly synthesized proteins are transported from the ER to the Golgi for modification and sorting to their destinations.
• The Golgi creates polysaccharides and packages proteins and lipids within vesicles for secretion or internal use.

Lysosome Functionality

• Lysosomes are membrane-bound organelles responsible for cellular waste degradation and recycling.
• Their acidic internal environment (pH 4.5-5.5) contains hydrolytic enzymes, facilitating the breakdown of internal and external materials.
• They play a role in autophagy, cellular repair, and can trigger autolysis when a cell is irreparably damaged.

Ribosome Structure and Function

• Ribosomes are essential for protein synthesis, directing what type of proteins are produced based on location.
• Composed of two subunits (large and small), they come together to facilitate protein production when needed.
• The ribosomal process involves initiation, elongation, and termination of protein synthesis.

Golgi Apparatus Structure

• The Golgi apparatus, made up of stacked pouches called cisternae, operates closely with the ER.
• Composed of the cis face (closest to the ER), trans face (farthest from the ER), and medial section, it processes proteins and lipids.
• The endoplasmic reticulum-Golgi intermediate compartment (ERGIC) facilitates transport between the ER and Golgi.

Golgi Processing Steps

• Proteins or lipids leave the ER as vesicles, enter the Golgi at the cis face, and are modified as they move through its lumen.
• Modifications may include sugar replacements or the addition of fatty acids or phosphates.
• At the trans face, molecules are sorted and packaged into vesicles for delivery to specific cellular locations.

This quiz explores the functions and structures of the smooth and rough endoplasmic reticulum (ER), highlighting their roles in lipid and protein synthesis, detoxification, and cellular regulation. Understand how these organelles differ in various cell types, including muscle and liver cells, and learn about lysosomal functions related to autophagy and heterophagy.